Urethane resin particles, which have excellent physical properties, have been widely used as resin particles for use in coating compositions, for example, to improve coating film performance.
Acrylic urethane composite resin particles have also been used in the field of coating compositions and the like, for example, to obtain the synergistic effects of acryl and urethane, improve compatibility when applied to acrylic resin-based coating compositions, and reduce the cost of urethane resin particles.
Synthesis of acrylic urethane composite resin particles can, based on the manner of dispersion in aqueous systems, be roughly categorized into (i) self-emulsification-type particles and (ii) particles dispersed using an emulsifier.
As an example of the self-emulsification-type particles, Patent Literature 1 discloses a method for producing an aqueous dispersion of a urethane-acrylic inner particle mixture with excellent film-forming properties and water resistance, wherein seed polymerization of an acrylic monomer is performed using an aqueous dispersion of urethane, that can be prepared without using an emulsifier, as a seed, and seed polymerization of an acrylic monomer is then performed again using the obtained aqueous dispersion of an inner particle mixture of urethane and acryl as a seed.
However, in such self-emulsification-type particles, it is necessary to introduce water-dispersible groups such as acid groups into the urethane resin portion, using a substantial amount of a water-dispersibility-imparting-group-containing compound in order to impart water dispersibility. Dispersion in and/or conversion to aqueous systems is achieved by virtue of water-dispersibility-imparting groups introduced into the urethane resin component. Thus, there may be the following problems: for example, the degree of flexibility in the composition, molecular weight, etc., of the urethane resin component is limited, and the production thereof is not easy in terms of equipment and dispersion conditions.
As an example of the particles dispersed using an emulsifier, Patent Literature 2 discloses a method for producing an aqueous dispersion of a (meth)acrylic composite resin by sequentially or continuously adding, to an aqueous medium, starting-material emulsions in which one or more (meth)acrylic monomers are dispersed beforehand to perform emulsion polymerization of the one or more (meth)acrylic monomers in the presence of a urethane resin.
However, in such a method of dispersion using an emulsifier, high shear force is necessary, thus requiring a special apparatus such as a high-pressure emulsifying apparatus in manufacturing equipment, or reducing the resistance to water load (e.g., water resistance and the like) of the obtained coating film when the particles are used for coating compositions because a substantial amount of an emulsifier is used. In addition, there is another problem such that the degree of flexibility in the composition (such as SP value) of the acrylic resin portion is limited.